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怎么理解 MySQL 中半同步引起 Master 实例 Crash，很多新手对此不是很清楚，为了帮助大家解决这个难题，下面丸趣 TV 小编将为大家详细讲解，有这方面需求的人可以来学习下，希望你能有所收获。
场景 :
Crash 发生时的数据库版本: MySQL-5.7.12, 官方标注在 5.7.17 进行了 fix;
开启半同步的主从架构中, 从库开启半同步, 启动 / 重启 slave 线程导致 Master 实例 Crash;
结论 :
mysql bug, 附上 bug 单链接: https://bugs.mysql.com/bug.php?id=79865
问题描述(摘抄):
Description: From 5.7，semi-sync add Ack_receiver thread for listening slave ack，which use select(). But select() can only listen socket fd between 1 and __FD_SET_SIZE(my os is 1024), when socket fd is bigger than __FD_SET_SIZE, select() has no effect, and can never get ack from slave,then semi-sync can t run normally.even more,select() use array store fds, when use FD_SET store fd which is bigger than __FD_SET_SIZE, array will overflow，so mysqld may crash。主要问题就出在 tcp 连接的 select 方法, 通常, 操作系统通过宏 FD_SET_SIZE 来声明一个进程中 select 能操作的文件描述符的最大数据, 然而通常情况下, 这个 FD_SET_SIZE 的值仅为 1024;
实际上, 用 epoll 或者 poll 会比较少, select 貌似是用的很少的;
问题复现:
准备一套 MySQL-5.7.12 的主从架构, 开启半同步:
为了能尽量简单的启用大量的文件描述符, 这里利用 MyISAM 分区表的 特性
这时候在主库上连续执行 select 语句多次(
这时候看一下主库的文件描述符数量;
那么现在在开启半同步的从库上重启一下 slave, 同时 tail 一下主库的日志;
在重启线程几秒钟之后, 主库就发生了 Crash;
PS: 在测试的过程中, 多次执行了 select 语句, 然后确认主库的半同步状态也是 ON 的情况下迅速在从库上重启 slave, 基本是必现的;
PPS: MyISAM 表在 open 的时候会同时打开所有的分区文件, 所以能比较方便的模拟占用大量文件描述符的情景;
(MyISAM 分区表: http://blog.itpub.net/29510932/viewspace-2134679/)
PPPPPPPS: _(:з」∠)_
附上测试用的脚本与 Crash 的信息
CREATE TABLE `myisam_t` (
`id` int(11) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4
/*!50100 PARTITION BY HASH (id)
PARTITIONS 2000 */
点击 (此处) 折叠或打开
2017-04-28T22:10:00.731611+08:00 5092 [Note] Start binlog_dump to master_thread_id(5092) slave_server(13043), pos(, 4)
2017-04-28T22:10:01.648365+08:00 5092 [Note] Start semi-sync binlog_dump to slave (server_id: 13043), pos(, 4)
*** buffer overflow detected ***: /usr/sbin/mysqld terminated
Backtrace:
/lib/x86_64-linux-gnu/libc.so.6(+0x731af)[0x7fcdfc7981af]
/lib/x86_64-linux-gnu/libc.so.6(__fortify_fail+0x37)[0x7fcdfc81dcf7]
/lib/x86_64-linux-gnu/libc.so.6(+0xf6f10)[0x7fcdfc81bf10]
/lib/x86_64-linux-gnu/libc.so.6(+0xf8c67)[0x7fcdfc81dc67]
/usr/lib/mysql/plugin/semisync_master.so(_ZN12Ack_receiver17get_slave_socketsEP6fd_set+0x83)[0x7fcc73d4a493]
/usr/lib/mysql/plugin/semisync_master.so(_ZN12Ack_receiver3runEv+0x603)[0x7fcc73d4aaf3]
/usr/lib/mysql/plugin/semisync_master.so(ack_receive_handler+0x19)[0x7fcc73d4aba9]
/usr/sbin/mysqld(pfs_spawn_thread+0x1b4)[0xe90784]
/lib/x86_64-linux-gnu/libpthread.so.0(+0x80a4)[0x7fcdfdf650a4]
/lib/x86_64-linux-gnu/libc.so.6(clone+0x6d)[0x7fcdfc80d87d]
点击 (此处) 折叠或打开
14:10:01 UTC – mysqld got signal 6 ;
This could be because you hit a bug. It is also possible that this binary
or one of the libraries it was linked against is corrupt, improperly built,
or misconfigured. This error can also be caused by malfunctioning hardware.
Attempting to collect some information that could help diagnose the problem.
As this is a crash and something is definitely wrong, the information
collection process might fail.
key_buffer_size=8388608
read_buffer_size=131072
max_used_connections=5
max_threads=9999
thread_count=8
connection_count=2
It is possible that mysqld could use up to
key_buffer_size + (read_buffer_size + sort_buffer_size)*max_threads = 21899362 K bytes of memory
Hope that s ok; if not, decrease some variables in the equation.
Thread pointer: 0x0
Attempting backtrace. You can use the following information to find out
where mysqld died. If you see no messages after this, something went
terribly wrong…
stack_bottom = 0 thread_stack 0x40000
/usr/sbin/mysqld(my_print_stacktrace+0x2c)[0xe77fec]
/usr/sbin/mysqld(handle_fatal_signal+0x459)[0x7a7019]
/lib/x86_64-linux-gnu/libpthread.so.0(+0xf8d0)[0x7fcdfdf6c8d0]
/lib/x86_64-linux-gnu/libc.so.6(gsignal+0x37)[0x7fcdfc75a067]
/lib/x86_64-linux-gnu/libc.so.6(abort+0x148)[0x7fcdfc75b448]
/lib/x86_64-linux-gnu/libc.so.6(+0x731b4)[0x7fcdfc7981b4]
/lib/x86_64-linux-gnu/libc.so.6(__fortify_fail+0x37)[0x7fcdfc81dcf7]
/lib/x86_64-linux-gnu/libc.so.6(+0xf6f10)[0x7fcdfc81bf10]
/lib/x86_64-linux-gnu/libc.so.6(+0xf8c67)[0x7fcdfc81dc67]
/usr/lib/mysql/plugin/semisync_master.so(_ZN12Ack_receiver17get_slave_socketsEP6fd_set+0x83)[0x7fcc73d4a493]
/usr/lib/mysql/plugin/semisync_master.so(_ZN12Ack_receiver3runEv+0x603)[0x7fcc73d4aaf3]
/usr/lib/mysql/plugin/semisync_master.so(ack_receive_handler+0x19)[0x7fcc73d4aba9]
/usr/sbin/mysqld(pfs_spawn_thread+0x1b4)[0xe90784]
/lib/x86_64-linux-gnu/libpthread.so.0(+0x80a4)[0x7fcdfdf650a4]
/lib/x86_64-linux-gnu/libc.so.6(clone+0x6d)[0x7fcdfc80d87d]
The manual page at http://dev.mysql.com/doc/mysql/en/crashing.html contains
information that should help you find out what is causing the crash.
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